This invention relates to a two-apron drafting unit for spinning machines comprising drivable bottom cylinders and pressure roller twins which are pressed against these bottom cylinders. The respective pressure roller twins are arranged with one shaft in the center of a pressure roller holder of at least one apron is guided by an apron cradle which, together with a load carrier, can be swivelled away from the assigned bottom cylinder. The apron cradle is provided with upper deflecting guides for the top aprons and, during the operation, is positioned by means of contact pressure, as a result of a spring force against at least two stops which are aligned with lower deflecting guides for bottom aprons which are arranged in parallel to the bottom cylinder. The first stop is used for determining the distance of the upper deflection guides with respect to the lower deflection guides, and the second stop is used for the parallel alignment of the upper deflection guides and of the pressure roller twin with respect to the lower deflection guides.
In the case of pressure roller twins and apron cradles, it is relatively difficult to ensure the parallel position of the apron cradles and of the pressure roller twins with respect to the bottom cylinders during the operation. There is the risk that, after an opening and closing of the drafting unit, the apron cradles each time take a changed position. For this reason, in the case of the known two-apron drafting units (German Utility Model 77 21 239 as well as U.S. Pat. No. 3,665,559), stops are provided for the parallel alignment of the apron cradles against which the apron cradles are placed without play under the effect of a spring when the drafting unit is closed. However, since normally the pressure roller holder, which is arranged in the load carrier and against which the loading spring is pressed, guides the shaft of the pertaining pressure roller twin virtually without play between guiding surfaces, it is necessary in the case of the state of the art to dispose the pressure roller holder as a whole to be displaceable against the stop in order to avoid redundancies. This is disadvantageous in that not only the shaft of the pressure roller twin guided by the apron cradle but, in addition, also the whole pressure roller holder must be aligned with respect to the apron cradle. In this case, the increased mass forces and friction forces counteract the endeavored goal.
It is also known (German Published Application 12 01 735) to load the apron cradle directly by means of the loading spring and to let a portion of the loading force act upon the shaft of the pressure roller twin. At the same time, the apron cradle is pressed against the lower deflection guide of the bottom apron in a spring-actuated manner. The parallel alignment should take place as a result of the fact that the shaft of the pressure roller twin is guided in a precisely axially parallel manner with respect to the pertaining bottom cylinder in the slot guides of the pressure roller holder. Because of the very narrow guiding base of the pressure roller holder, however, a sufficient parallel alignment is not possible.
In an older German Patent Application (DE 42 15 004.3), which is no prior publication, it was suggested to use a common apron cradle for the top apron and the bottom apron and to place it in a swivellable manner on the bottom cylinder. In this case, the apron cradle is supported against a stop so that the pressure roller twin is aligned in parallel to the bottom cylinder by means of the apron cradle. In contrast to this suggestion, the novelty of the object of the application is the fact that deflection guides are provided for the bottom aprons which are independent of the bottom cylinder.
It is an object of the invention to permit, in the case of a two-apron drafting unit of the initially mentioned type, a good parallel alignment of the upper deflection guides and of the pertaining pressure roller twin with respect to the bottom cylinders without the requirement of also aligning the pressure roller holder by means of the apron cradle.
This object is achieved in that the shaft of the pressure roller twin is unguided in the pressure roller holder in the radial direction.
As a result of the characteristics of the invention, the pressure roller holder essentially only receives the function of providing the load as well as, if required, the lateral guiding, while the parallel alignment of the apron cradle and of the pressure roller twin is determined by at least one stop, against which the apron cradle is placed under the effect of a spring without any clearance and, in the process, will guide the pressure roller twin. The pressure roller holder itself will not go along in these movements against the stop. As a result, in comparison to the state of the art, the mass forces and friction forces are reduced significantly so that the precision of the parallel alignment is increased. By means of the apron cradle, which reaches around the shaft, the pressure roller twin is also pressed against the assigned bottom cylinder without any play.
In a development of the invention, it is provided that the spring-loaded pressure roller holder or its loading spring is applied directly to the apron cradle by means of a force introducing surface which generates a force component against both stops. Deviating from the state of the art, the loading force is not transmitted directly by the pressure roller holder to the shaft of the pressure roller twin, but the pressure roller holder first loads the apron cradle which transmits its load to the shaft. In this case, a component of this load acts directly against the stops. In this manner, as few masses as possible have to be moved.
However, as an alternative, the pressure roller holder or its loading spring may also be applied directly to the shaft of the pressure roller twin by means of a sliding surface which permits radial movements of the shaft.
Expediently, the pressure roller holder is provided with guiding surfaces which guide the apron cradle laterally. They limit the mobility of the apron cradle in the lateral direction.
In a particularly advantageous development of the invention, it is provided that at least two stops each have a stop face, the two stop faces changing into one another in an approximately L-shaped manner. This has the advantage that the upper deflection guides can virtually simultaneously be placed against both stops. The L-shaped profile of the stop faces ensures that the spacing of the upper deflection guides with respect to the lower deflection guides may possibly take place by means of a single component.
Expediently, a sliding surface is assigned to the stop faces which leads the apron cradle to the stops. As a result, it is ensured that, when the drafting unit is closed, the apron cradle automatically finds its operating position.
Expediently, at least one of the stops is exchangeably mounted on the assigned lower deflection guide which is preferably constructed as a reversing rail. As a result, the drafting unit can be adapted to different fiber materials to be spun. For example, by means of this measure, the distance of the upper deflection guide to the pertaining lower deflection guide, the so-called opening width, can be adjusted. This distance of the forward deflection of the apron pairs with respect to the nip line of the roller pair that follows can also be adjusted in this manner.
Advantageously, the lower deflection guides can be adjusted with respect to their position relative to the drafting zone plane of the drafting unit. It therefore becomes possible not only to adjust the desired opening width, but also to carry out this adjustment symmetrically with respect to the drafting zone plane.
In an advantageous development of the invention, the second stop, which is used for the parallel alignment, comprises two partial stops whose distance from one another corresponds approximately to the distance of the top aprons pertaining to a pressure roller twin. As a result, a very wide guiding base of those stops is achieved which take over the parallel alignment. The larger the guiding base, the more exactly the pressure roller twins can be aligned with respect to the pertaining bottom cylinder.
In a further development of the invention, a holding device is assigned to the second stop which holds a sliver guide. Consequently, the so-called condenser can also be adjusted precisely with respect to the apron cradle and the roller pair which follows.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.